Monitoring apparatus for preventing spark-overs at a high voltage installation

ABSTRACT

A monitoring apparatus for preventing a spark-over at a high voltage installation for corona electrodes comprising a highfrequency voltage source and a high voltage transformer, the secondary winding of which is coupled through the agency of a high voltage cascade with the corona electrode. The highfrequency voltage source is connected via a power amplifier, which starting from a certain value of its output current exhibits current limiting and decreasing output voltage characteristics, with the primary winding of the high voltage transformer. A comparator has its inputs coupled with the input and output respectively of the power amplifier and in the event the output voltage of the power amplifier is smaller than its input voltage interrupts the current circuit to the corona electrode through the agency of a cutoff mechanism.

United States Patent 11 1 [111 3,764,883

Staad et al. Oct. 9, 1973 MONITORING APPARATUS FOR 3,217,207 11/1965Webb 317/51 PREVENTING SPARK-OVERS AT A HIGH i g i i ams... VOLTAGEINSTALLATION 3,665,253 5/1972 Stefani 317/31 X [75] Inventors: FranzBraun Staad; Peter Ribnitz,

both of St. Gallen, Switzerland Primary Examiner-William M. Shoop, Jr.[73] Assignee: Gema AG Apparatebau, St. Gallen, AttorneymKarl mocksSw1tzerland ABSTRACT [22] Filed: 1972 A monitoring apparatus forpreventing a spark-over at [21] AppL No; 301,149 7 V a high voltageinstallation for corona electrodes comprising a high-frequency voltagesource and a high voltage transformer, the secondary winding of which isFmeign Application Priority Data coupled through the agency of a highvoltage cascade Oct. 28, 1971 Switzerland 15725/71 with the coronaelectrode. The high-frequency voltage source is connected via a poweramplifier, which start- [52] US. Cl 321/11, 204/312, 250/324, ing from acertain value of its output current exhibits 317/27, 330/51 currentlimiting and decreasing output voltage charac- [51] Int. Cl. H02m 1/18teristics, with the primary winding of the high voltage [58] Field ofSearch 204/312, 317, 327; transformer. A comparator has its inputscoupled with 250/324, 325, 326; 317/27 R, 31, 51; the input and outputrespectively of the power ampli- 330/51; 321/11 fier and in the eventthe output voltage of the power amplifier is smaller than its inputvoltage interrupts [56] References Cited the current circuit to thecorona electrode through the UNITED STATES PATENTS agency of a cutoffmechanism.

2,066,522 H1937 Doherty 330/51 6 Claims, 2 Drawing Figures 2 25 15 1asc/mroe E g M64 VGZTAGE .7 5456x9092 0156/1015 5 TEA/YSRJR/If? 7MONITORING APPARATUS FOR PREVENTING SPARK-OVERS AT A HIGH VOLTAGEINSTALLATION BACKGROUND OF THE INVENTION The present invention relatesto a new and improved construction of monitoring apparatus forpreventing spark-overs at a high voltage installation, especially forcorona electrodes, with a high-frequency voltage source and a highvoltage transformer, the secondary winding of which is coupled throughthe agency of a high voltage cascade circuit arrangement with the coronaelectrode.

Prior art apparatuses for monitoring the high voltage at coronaelectrodes in equipment for spray painting or lacquering are only thengenerally effective when there has already occurred a spark-over owing,for instance, to the corona electrode and the object undergoingtreatment approaching one another too closely. A great prevalent dangeris the possible explosion of the sprayed powder cloud. Additionally,during each spark-over the surface of the sprayed coating is impaired.Therefore attempts have been made to reduce to the greatest possibleextent the energy of the sparkover by employing an alternating'currentvoltage of as high as possible frequency, so that the high voltagetransformer and the capacitances at the high voltage cascade circuitarrangement can be selected to be small and thus store very littleenergy. Other monitoring devices contemplate reducing the electrodevoltage as soon as a spark-over condition arises.

SUMMARY OF THE INVENTION It is therefore a primary object of the presentinvention to provide a new and improved construction of monitoringapparatus which effectively prevents sparkovers at a high voltageinstallation.

Another and more specific object of the present invention relates to anew and improved construction of monitoring apparatus which isrelatively simple in construction and design and which removes thevoltage from the corona electrode before a sparkover condition arises.

Now in order to implement these and still further objects of theinvention, which will become more readily apparent as the descriptionproceeds, the inventive monitoring apparatus of this development ismanifested by the features that the high-frequency voltage source isconnected via a power amplifier, which starting from a certain value ofits output current exhibits current limiting and decreasing outputvoltage characteristics, with the primary winding ofa high voltagetransformer. A comparator has its inputs connected with the input andoutput respectively of the power amplifier and in the event that theoutput voltage of the power amplifier is smaller than its input voltagethe comparator, through the agency of a cutoff mechanism, interrupts thecurrent circuit to the corona electrode.

BRIEF DESCRIPTION OF THE DRAWING The invention will be better understoodand objects other than those set forth above, will become apparent whenconsideration is given to the following detailed description thereof.Such description makes reference to the annexed drawing wherein:

FIG. 1 is a schematic circuit diagram of monitoring apparatus designedaccording to the teachings of this development wherein a portion of thecircuit diagram is depicted in block configuration and another portionthereof depicted with its individual components; and

FIG. 2 is a graph showing the dependency of the output voltage of thepower transformer upon the output current with constant input voltage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Describing now thedrawing, in FIG. 1 there is illustrated an exemplary embodiment of theinventive monitoring apparatus embodying a high-frequency voltage sourcein the form of an oscillator 1 which generates an alternating-currentvoltage at a fequency of 17 kHz with a constant voltage of 12 volts. Theoutput of the oscillator 1 is electrically coupled with a first terminal20 of a first potentiometer R,, the second terminal 21 of which isgrounded. The sliding tap 30 of potentiometer R is coupled through theagency of a rest contact a belonging to a relay A defining a cutoffmechanism with the input 22 of a known power amplifier 2 and with theanode 23 of a first diode D,. This power amplifier 2 possesses a largecurrent gain; its voltage gain normally amounts to approximately 1. Theoutput voltage U appearing at the amplifier output 24 corresponds to theinput voltage U, when the output current la does not exceed a certainvalue. On the other hand, if the output current I exceeds the value lathen the output voltage U, decreases approximately in linear fashion, asbest seen by referring to FIG. 2, and amounts to zero at the time ofmaximum possible output current Ia" (short-circuit condition). Theoutput 24 of the amplifier 2 is coupled with the primary winding 25 of ahigh-voltage transformer 3 and with the anode 26 of a second diode D Thesecondary winding 27 of the high-voltage transformer 3 is coupled withthe input 28 of a high-voltage cascade circuit arrangement 4 whichdelivers a direct-current voltage of about kV to a corona electrode 5.

The first terminal 31 of a second potentiometer R and a terminal 32 of acapacitor C the other terminal 33 of which is grounded, are connectedwith the cathode 34 of the first diode D The second terminal 35 of thepotentiometer R is connected to ground via a resistor R The sliding tap40 of the potentiometer R is connected to the non-inverting input 41,also designated by reference character with a comparator 6. The cathode29 of the second diode D is connected with the inverting input 42, alsodesignated by reference character of the comparator 6 and a terminal 43of a grounded capacitor C The base 44 of a switching transistor T iscoupled through the agency of a resistor R, with the output 45 of thecomparator 6. At the collector current circuit of transistor T there isar ranged the relay A.

With the foregoing background in mind there will be now considered themode of operation of the previously described monitoring apparatus.During normal operation, that is to say, when the output current I ofthe amplifier is smaller than Ia then the voltage U which corresponds tothe rectified output voltage Ua and is applied to the invertingcomparator input 42, is more positive than the voltage U, at thenon-inverting comparator input 41. The voltage U corresponds to therectified amplifier input voltage U,,, but attenuated by the combinationof the potentiometer R and resistor R The output voltage U, of thecomparator 6 is negative if the voltage U applied to the inverting input42 is more positive than at the non-inverting input 41 of the comparator6. The switching transistor T in the normal condition is thereforenon-conductive, the relay A in its rest state, and the rest contact a isclosed.

Before a spark-over appears at the corona electrode 5 the electrodecurrent and accordingly also the current at the primary winding 25 ofthe high voltage transformer, which approximately corresponds to theoutput current Ia of the amplifier 2, begins to rise. Upon exceeding thevalue Ia, which is dependent upon the design of the amplifier, theoutput voltage Ua begins to drop and therefore also the voltage U In theevent that the voltage U becomes more negative than the voltage U,, thenthe output voltage of the comparator 6 becomes positive, the transistorT conducts, the relay A is actuated and the rest contact a opens, sothat the current supply to the corona electrode 5 is interrupted.

With the aid of the potentiometer R and the resistor R it is possible toregulate the relationship of the amplifier input voltage to theamplifier output voltage, at which the monitoring apparatus is torespond. The resistor R serves the purpose of preventing the voltage Ufrom dropping to zero. If this should happen then the equipment will beerroneously completely disconnected so that even with a spark-over itwill not respond.

With the aid of the potentiometer R it is possible to regulate the inputvoltage Ue of the amplifier and therefore indirectly also the highvoltage of the corona electrode. An advantage of this monitoring deviceis that it is not dependent upon the adjusted value of the input voltageUe rather upon the relationship between the amplifier input voltage tothe amplifier output voltage. In this way there can be realized apositive cutoff or disconnection whenever the corona electrode voltagehas dropped to a certain percentage of a reference value.

While there is shown and described present preferred embodiments of theinvention, it is to be distinctly understood that the invention is notlimited thereto, but may be otherwise variously embodied and practicedwithin the scope of the following claims. ACCORD- INGLY,

What is claimed is:

l. A monitoring apparatus for preventing spark-overs at a high voltageinstallation for corona electrodes, comprising a high-frequency voltagesource, a corona electrode, a high-voltage transformer having a primarywinding and a secondary winding, a high-voltage cascade arrangementconnected in circuit with the corona electrode, the secondary winding ofsaid high-voltage transformer being coupled with the corona electrodevia said high-voltage cascade arrangement, a power amplifier means,which starting at a certain value of its output current exhibits currentlimiting and decreasing output voltage characteristics, saidhigh-frequency voltage source being electrically coupled with theprimary winding of the high-voltage transformer through the agency ofsaid power amplifier means, comparator means having a first input, asecond input and an output, said power amplifier means having an inputand an output, one of the inputs of said comparator means beingconnected with the input of the power amplifier means and the otherinput of said comparator means being connected with the output of saidpower amplifier means, cutoff means for the corona electrode, saidcomparator means interrupting the current circuit to the coronaelectrode via said cutoff means when the output voltage of the poweramplifier means is smaller than its input voltage.

2. The monitoring apparatus as defined in claim 1, wherein thehigh-frequency voltage source comprises oscillator means with adjustableoutput voltage.

3. The monitoring apparatus as defined in claim 1, further including afirst rectifier arranged between the input of the power amplifier meansand said one input of the comparator and a second rectifier arrangedbetween the output of said power amplifier means and said other input ofsaid comparator.

4. The monitoring apparatus as defined in claim 1, further includingpotentiometer means electrically coupled between the input of the poweramplifier means and said one input of the comparator means for adjustingthe relationship between the amplifier input voltage to the amplifieroutput voltage at which the monitoring apparatus responds independent ofthe value of the amplifier input voltage.

5. The monitoring apparatus as defined in claim 1, wherein said cutoffmeans comprises a relay which can be actuated by said comparator means,said relay having a switching contact arranged in circuit between thehigh-frequency voltage source and the power amplifier means.

6. The monitoring apparatus as defined in claim 1, wherein the currentvalue at which there occurs limiting of the output current can beadjusted by changing two components at the power amplifier means.

' UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTEON PatentNo.3,764,883 Dated October 9, 1973 l fi srk Franz Braun and Peter RibnitzIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

[75] l'nventorsz Franz Braun, Staad; Peter Ribnitz, St. Gallen,

' Switzerland Signed and sealed this 19th day of February 197M.

(SEAL) Attest;

EDWARD M.FLE TCHER,JR. MARSHALL DANN Attesting Officer Commissioner ofPatents FORM PO-105O (10-69) USCOMM-DC GOING-P69 u.s. sovuunzn-r rnm'rmeornc: ID" o-acd-au,

w U Wm v .-w ;w

I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent: No; 3764 5 883 D g d October. 9 1973 Inveha g Franz Braun' and Peter Ribnit zIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

[75] Inventors-Q Franz Braun, Staad; Peter Ribnitz, St. Gallen,

' Switzerland Signed and sealed this 19th day of February 197A.

SEAL) Attest;

c. MARSHALL DANN Commissioner of Patents EDWARD MIFLETQCHER, JR. Attesting Officer FORM PO-105O (10-69) uscoMM-Dc coon. U. S. GOVIINIIENTPRINTING OF"; 1

1. A monitoring apparatus for preventing spark-overs at a high voltageinstallation for corona electrodes, comprising a highfrequency voltagesource, a corona electrode, a high-voltage transformer having a primarywinding and a secondary winding, a high-voltage cascade arrangementconnected in circuit with the corona electrode, the secondary winding ofsaid high-voltage transformer being coupled with the corona electrodevia said high-voltage cascade arrangement, a power amplifier means,which starting at a certain value of its output current exhibits currentlimiting and decreasing output voltage characteristics, saidhigh-frequency voltage source being electrically coupled with theprimary winding of the high-voltage transformer through the agency ofsaid power amplifier means, comparator means having a first input, asecond input and an output, said power amplifier means having an inputand an output, one of the inputs of said comparator means beingconnected with the input of the power amplifier means and the otherinput of said comparator means being connected with the output of saidpower amplifier means, cutoff means for the corona electrode, saidcomparator means interrupting the current circuit to the coronaelectrode via said cutoff means when the output voltage of the poweramPlifier means is smaller than its input voltage.
 2. The monitoringapparatus as defined in claim 1, wherein the high-frequency voltagesource comprises oscillator means with adjustable output voltage.
 3. Themonitoring apparatus as defined in claim 1, further including a firstrectifier arranged between the input of the power amplifier means andsaid one input of the comparator and a second rectifier arranged betweenthe output of said power amplifier means and said other input of saidcomparator.
 4. The monitoring apparatus as defined in claim 1, furtherincluding potentiometer means electrically coupled between the input ofthe power amplifier means and said one input of the comparator means foradjusting the relationship between the amplifier input voltage to theamplifier output voltage at which the monitoring apparatus respondsindependent of the value of the amplifier input voltage.
 5. Themonitoring apparatus as defined in claim 1, wherein said cutoff meanscomprises a relay which can be actuated by said comparator means, saidrelay having a switching contact arranged in circuit between thehigh-frequency voltage source and the power amplifier means.
 6. Themonitoring apparatus as defined in claim 1, wherein the current value atwhich there occurs limiting of the output current can be adjusted bychanging two components at the power amplifier means.